Intensity refers to load/weight and has been shown to have a significant impact on muscle hypertrophy and is arguably the most important exercise variable for stimulating muscle growth. Intensity can be recorded as a percentage of 1RM and equates to the number of repetitions that can be performed with a given weight.
Repetitions can be classified into 3 basic ranges:
- low (1–5)
- moderate (6–12)
- high (15+)
These repetition ranges involve the use of different energy systems and stress the neuromuscular system in different ways. This variation of stress impacts the extent of the hypertrophic response.
The use of high repetitions has generally proven to be inferior to moderate and lower repetition ranges in eliciting increases in muscle hypertrophy. This means that a load less than approximately 65% of 1RM is not considered sufficient to promote substantial hypertrophy. This is because the high rep training can bring about significant metabolic stress but the load is inadequate to recruit and fatigue the highest threshold muscle fibres.
Whether low or moderate reps evoke a greater hypertrophic response has been a matter of debate as both produce significant gains in muscle growth. However, the general consensus is that the moderate rep range of 6–12 reps will optimise muscle hypertrophy.
The Moderate Rep Range
The moderate rep range has an anabolic superiority because of factors associated with metabolic stress. Although low rep sets are carried out almost exclusively by the phosphocreatine system, moderate reps rely heavily on anaerobic glycolysis which results in a significant buildup of metabolites. Studies of bodybuilding routines performed with multiple sets of 6–12 reps show significant post-exercise declines in ATP, creatine phosphate and glycogen, along with marked increases in blood lactate, intramuscular lactate, glucose and glucose-6-phosphate. It’s the buildup of these metabolites that has been shown to have a significant impact on anabolic processes.
Therefore it seems that there is a maximum threshold for tension-induced hypertrophy above which metabolic factors become more important than additional increases in load. Because of this metabolic buildup, moderate rep range training has been shown to maximize the acute anabolic hormonal response of exercise. Both testosterone and growth hormone are acutely elevated to a greater degree from routines employing moderate rep sets as compared to those using lower reps, thereby increasing the potential muscular hypertrophy.
Chasing The “Pump”
Training in the moderate rep range also maximizes acute cellular hydration. During moderate rep training, the veins taking blood out of working muscles are compressed while arteries continue to deliver blood into the working muscles, thereby creating an increased concentration of intramuscular blood plasma. This is what bodybuilders call the “pump”. The “pump” causes plasma to seep out of the capillaries and into the interstitial spaces. The buildup of fluid in the interstitial spaces causes an extracellular pressure gradient, which causes a flow of plasma back into the muscle causing the “pump” phenomenon. This “pump” is augmented by the accumulation of metabolic byproducts, which function as osmolytes, drawing fluid into the cell.
It’s not really known whether this exercise-induced cell swelling helps with muscle hypertrophy, but it seems plausible given the known role of hydration in regulating cell function. Also, the extra time-under-tension associated with a moderate rep scheme would enhance the potential for microtrauma and fatigue across the full spectrum of muscle fibers. This would benefit the hypertrophy of slow-twitch fibers, which have greater endurance capacity than fast-twitch fibers. Although slow-twitch fibers are not as responsive to growth as fast-twitch fibers, they still do display hypertrophy when subjected to an overload stimulus. Given that the majority of muscles have slow-twitch fibres, the moderate rep protocol will stimulate all muscle fibres and result in complete muscle growth.
For information sake we should note that some opinions suggest that muscles containing a greater percentage of slow-twitch fibers might have the greatest hypertrophic response to a higher rep range, whereas fast-twitch muscles would respond best to lower rep. Although this concept sounds logical, a muscle fiber-type prescription with respect to rep range has not been proven by research. Given the variability of fiber-type composition between individuals, it would be difficult if not impossible to determine fiber-type ratios without a muscle biopsy, thus making the application of this idea impractical for the vast majority of people.